1. Field of the Invention
The present invention relates to a wafer sheet, a method of producing a semiconductor device using the wafer sheet, and an apparatus for producing a semiconductor device. More particularly, it relates to a semiconductor wafer laminated on its back side to a wafer sheet which is covered by thermosetting adhesive layers on both sides while element separation and die bonding operations are carried out substantially simultaneously, thereby improving the productivity, a method of producing a semiconductor device using the wafer sheet, and an apparatus for producing a semiconductor device.
2. Description of the Related Art
Steps for producing semiconductor devices include a bonding step. The bonding step comprises a die bonding step where each of semiconductor elements (dice) which have been separated from a wafer is bonded onto a die pad and a wire bonding step where electrodes on the semiconductor element 3 and inner leads are electrically connected with each other by means of thin metal wires.
As the wire bonding operation, among the operations in the bonding step, has been increasingly automated, it has become relatively more important to rationalize the feeding of semiconductor elements around a die bonder. As a result, various methods of feeding the semiconductor elements, such as a direct pickup system, have recently been devised by manufacturers and have been disseminating.
FIGS. 10A, 10B are schematic diagrams showing a die bonding apparatus with the direct pickup system of the prior art, where FIG. 10A is a perspective view and FIG. 10B is a cross sectional view of a key portion.
As shown in the drawing, while locating the semiconductor element 3 attached to the wafer sheet 2, only good elements are thrust up by a thrust pin 38, picked up by a suction head 39, moved onto a die pad 30 of a lead frame 29 and bonded thereon. Good elements may be identified either by using an ink mark provided on the surface of a defective element or by means of a map of good elements stored on a floppy disk (not shown and hereinafter referred to as F/D) which provides the information on the positions of good elements.
However, either of the methods described above has such problems that, since the selected elements are thrust up by the thrust pin 38, picked up by the suction head 39, moved onto the die pad 30 which is located at a distance and bonded thereon, there is a possibility that the semiconductor element 3 is damaged during thrusting, picking up or die bonding, and the throughput decreases due to longer time required for the bonding step as a whole. In an attempt to solve the problems of the prior art, for example, Japanese Patent Laid-Open Publication No. Hei. 6-204267 discloses a semiconductor wafer having an adhesive tape attached on the front side and is diced on the back side each. of the separated semiconductor elements is bonded on the back side to a element mounting position of a lead frame by pressing on the front surface of the semiconductor element via the adhesive tape using a die bonding fixture, and the adhesive tape is removed and the semiconductor element is bonded on the back side to the element mounting position in a single step.
However, with the die bonding step described above, although the adhesive tape is attached to the front surface of the semiconductor wafer, possibility of damaging the integrated circuit due to mechanical impact cannot be eliminated because the front surface of the semiconductor element the integrated circuit is formed is pressed via the adhesive tape by the die bonding fixture. Further, since the adhesive tape is attached to the front surface of the semiconductor wafer, an adhesive material sticks onto the surface of electrodes provided on the front surface of the wafer thereby causing contamination and, wire bonding of thin metal wires thereon without cleaning may result in poor reliability of connection. To solve this problem, it is required to introduce a new process of cleaning the electrode surface.
The present invention has been attained to solve the problems of the die bonding method of the prior art as described above, and an object of the present invention is to provide a wafer sheet surely carrying out die bonding with fewer steps without causing damage to the semiconductor elements, thereby improving productivity, a method of producing a semiconductor device using the wafer sheet, and a semiconductor producing apparatus.
The wafer sheet according to the present invention comprises an expandable resin sheet with thermosetting adhesive layers formed on both sides and a semiconductor wafer attached to the back side thereof before dicing.
The present invention also provides a wafer sheet wherein the expandable resin sheet is made of soft vinyl chloride. Further the present invention provides a wafer sheet wherein the expandable resin sheet is made of a polyimide-modified epoxy resin containing 40% by weight of a silver filler.
Further the present invention provides a wafer sheet wherein the expandable resin sheet is made of an electrically conductive sheet with electrically conductive thermosetting adhesive layers formed on both sides thereof.
The present invention also provides a wafer sheet wherein the expandable resin sheet has multitude of through holes.
The method of producing the semiconductor devices according to the present invention comprises the steps of bonding one side of the wafer sheet made from the expandable resin sheet with thermosetting adhesive layers formed on both sides thereof laminated onto the back side of the semiconductor wafer, and dividing the semiconductor wafer into a plurality of semiconductor elements by dicing; expanding the wafer sheet which carries the separated semiconductor elements being laminated thereon, thereby widening the separation grooves between the semiconductor elements; positioning proper one among the semiconductor elements on a die pad of a lead frame whereon the element is to be die-bonded while being separated from each other; and cutting off the wafer sheet below the expanded separation grooves surrounding the semiconductor elements which have been positioned on the die pads thereby separating the piece of wafer sheet with the semiconductor element laminated thereon, and pressing the adhesive surface on the side of the wafer sheet, opposite to that where the semiconductor element is attached, onto the die pad surface.
The present invention also provides a method wherein the separated semiconductor element is pressed by means of compressed air thereby pressing the adhesive surface on the side of the wafer sheet opposite to that where the semiconductor element is attached onto the die pad surface.
The semiconductor device producing apparatus of the present invention comprises a stage whereon a wafer sheet carrier, which holds the wafer sheet with the semiconductor elements laminated thereon while being separated from each other, is placed at a distance from the lead frame; a die bonding means which is disposed at a die bonding position of the stage to be capable of moving vertically and has a hollow space to cover the semiconductor element while keeping a specified clearance during die bonding, a die bonding head having a compressed air passage opening at the bottom of the hollow space and a cutting blade for cutting off the wafer sheet below the expanded separation grooves between the semiconductor elements provided at the tip of the die bonding head and a drive section for driving the die bonding head; and a compressed air supplying means for supplying compressed air into the compressed air passage during die bonding of the semiconductor element.
The present invention also provides a semiconductor device producing apparatus wherein the cutting blade is disposed to be inserted through the expanded separation grooves with a specified clearance kept from the side faces of the grooves surrounding the semiconductor element to be die-bonded, in such a way that the cutting blade can be attached to the tip of the die bonding head and removed therefrom.
The present invention also provides a semiconductor device producing apparatus provided with X-/Y-axis direction drive means for moving the stage in X- and Y-axis directions thereby to move a proper element among the semiconductor elements, which have been separated from each other, onto a die pad of the lead frame whereon the element is to be die-bonded and setting the element thereon, and element rotating means for rotating the wafer sheet carrier placed on the stage thereby to adjust the inclination angle of the semiconductor element at the setting position thereof.
Also according to the present invention, the semiconductor device producing apparatus has the compressed air passage of the die bonding head disposed on the vertical line passing through the center of the semiconductor element to be die-bonded, a semiconductor layer disposed on the die bonding head to emit a light beam in the vertical direction toward the semiconductor element surface, a element position detecting means for locating the point on the semiconductor element surface which is irradiated by the laser beam emitted by the semiconductor laser and generating a position signal to indicate deviation of the semiconductor element from the normal position in the X-axis and Y-axis directions with zero position being set at the laser spot and the information on the inclination angle from the normal direction, and a die bonding controller which, upon receipt of the position signal from the element position detecting means, sends correction signals for the displacement or inclination angle to the X-/Y-axis direction drive means or the element rotating means and, when the position of the semiconductor element is corrected, sends an operation command signal to the die bonding means and the compressed air supplying means to start the specified operations.
The present invention, having the constitution described above, has such effects as described below.
Since the wafer sheet is made from the expandable resin sheet with thermosetting adhesive layers formed on both sides thereof, the semiconductor elements laminated on the wafer sheet can be separated into individual elements and die-bonded substantially at the same time during dicing.
By using the soft vinyl chloride for the expandable resin sheet, low-cost wafer sheet can be obtained.
By using the expandable resin sheet made of a polyimide-modified epoxy resin containing 40% by weight of a silver filler, the wafer sheet having good heat conductivity can be obtained without significantly affecting the stretching capability.
Since both the expandable resin sheet and the thermosetting adhesive layers formed on both sides thereof are made of electrically conductive materials, the wafer sheet is capable of accommodating the semiconductor element which requires it to make the potential of the back surface of the semiconductor element equal to the ground level.
Since the expandable resin sheet has multitude of through holes, the resin sheet is capable of stretching more, and therefore wider expanded separation groove can be obtained during expansion of the wafer sheet, thus making it possible to insert the cutting blade of the die bonding head into the expanded separation groove with a greater margin and making it easier to separate the semiconductor elements into individual devices.
Further, since the back side of the semiconductor wafer element is laminated onto one of the adhesive layers of the wafer sheet and, after expanding the separation grooves made by dicing, the wafer sheet below the expanded separation groove is cut off thereby separating the semiconductor elements into individual devices while the semiconductor element is pressurized to have the other adhesive surface of the wafer sheet laminated onto the die pad surface, the semiconductor element can be die-bonded with less number of steps and the productivity is improved.
Also since the semiconductor element is pressed by compressed air during die bonding, there is no possibility of causing mechanical damage to the semiconductor element.
Further, since the semiconductor device producing apparatus is made in a compact configuration comprising the stage whereon the wafer sheet is placed, the die bonding means having the compressed air passage, the cutting blade which cuts off the wafer sheet below the expanded separation groove between the semiconductor elements and the compressed air supplying means which supplies compressed air to the compressed air passage, the compact semiconductor device producing apparatus can be provided at a low cost wherein separation of the semiconductor elements into individual devices and die bonding operation can be carried out substantially at the same place and substantially at the same time.
Also since the cutting blade which cuts off the wafer sheet below the expanded separation groove is made in a size corresponding to the size of the semiconductor device to be die-bonded is provided to be attached to and detached from the bonding head, different kinds of the semiconductor device can be accommodated very easily and quickly.
Further, since the X-/Y-axis direction drive means of the stage whereon the wafer sheet carrier is placed and the element rotating means are provided, displacement and inclination of the semiconductor element to be die-bonded can be easily corrected.
Also since the semiconductor producing apparatus is made in such a configuration that is provided with the element position detecting means which issues the position signal representing the displacement and inclination in the setting position of the semiconductor element to be die-bonded, and the die bonding controller which issues the correction signal based on the position signal to control the X-/Y-axis direction drive means and the element rotating means of the stage whereon the wafer sheet carrier is placed, the semiconductor element remaining on the wafer sheet can be easily moved to the normal bonding position by correcting the displacement and inclination.